This invention generally relates to electronic systems and in particular it relates to a comparator input stage.
One of the main problems with prior art comparators is the large common mode variation at the input stage of the comparator. This stage needs to support a common mode of 0.6V to 2.1V, with a differential swing as low as +/xe2x88x9225 mV. The next important point is the large variation in the supply voltage that the comparator should tolerate. For example, the supply voltage can vary from 2.7V to 3.6V.
To support the large common mode at the input (in the 1.8 v/3.3 v process) the supply must be 3.3 v. Hence, conventionally 3.3V transistors would be used as the input pair. Such transistors have a large threshold voltage (Vt), which prevents the N channel input pair from supporting a low common mode of 0.6V. If 1.8V transistors are instead used as the input pair, a lower common mode may be achieved, but the circuit shows reliability problems due to the use of a 3.3V supply.
In one prior art method used to solve the above problem, the input pair consists of two pairs of 3.3V transistors. One of the pairs consists of N channel input transistors, and the other pair consists of P channel input transistors. Depending on the common mode, the correct pair comes into operation. The problem with this method is that an extra comparator for every input is required.
Another prior art method is to use 1.8V N-channel input pair transistors with a resistor in the common current path, which drops a suitable amount of voltage across it. This method has its difficulties in this specific case because of the large power supply variation (the supply itself varies by 0.9V). This in effect means that it is impossible to determine the correct amount of voltage to be dropped across this resistor. If this dropped voltage is selected to be too small (for example xcx9c0.4V or less), then for the supply voltage of 3.6V, there is still a reliability problem. On the other hand, if a larger value is dropped across the resistance, a curtailed common mode range is a problem at the higher end for a 2.7V supply voltage. Also, the output common mode of this stage is a function of the supply voltage and thus varies greatly.
Another prior art solution is a regulator with a 1.8V N-channel input pair. The regulator (closed loop, with an op-amp) is used to generate a suitable voltage (for example 2.4V) which is such a value as to not cause the reliability problem but at the same time not limit the upper side of the common mode range. This voltage is used as the supply for the comparator. However, this method requires the design of a dedicated regulator in closed loop for the comparator.
A comparator input stage uses low voltage transistors 20 and 21 as the input pair. They have a small threshold voltage, and hence support a low common mode. The circuit includes a current sink 22 coupled to the input pair 20 and 21; a first resistor 33 coupled between a first branch of the input pair and a voltage node V24; a second resistor 36 coupled between a second branch of the input pair and the voltage node V24; a first transistor 23 coupled to the voltage node V24; a second transistor 24 having a gate coupled to a gate of the third transistor 23; a third resistor 32 coupled to a first end of the second transistor 24; and a current source 29 coupled to a second end of the second transistor 24 for controlling a voltage across the third resistor 32 wherein the voltage across the third resistor 32 sets a voltage at the voltage node V24. This voltage at the voltage node V24 serves as an open loop regulation for protection of the input pair transistors 20 and 21.